Recently, a novel family of alloys with exceptional permanent magnetic strength were invented. These alloys are based on light rare earth elements (RE), preferably neodymium and praseodymium; the transition metal element, iron; and boron. The primary phase of the magnetic alloys is believed to have the composition RE.sub.2 Fe.sub.14 B, while the preferred composition of the starting alloy is in the range of about RE.sub.0.12-015 B.sub.0.04-0.09 Fe.sub.bal (atomic fractions). These alloys are also known under the General Motors tradename "MAGNEQUENCH".
A preferred method of processing such alloys to make magnets is melt-spinning. Melt-spinning entails casting a stream of molten alloy onto the perimeter of a rotating chill disk to very rapidly quench the alloy into thin ribbon. The rate of solidification is controlled by regulating the wheel speed to create magnetic domain or smaller sized crystallites in the ribbons as quenched. Rapidly quenched alloy with subdomain sized crystallites may be heated to suitable temperatures to cause grain growth to optimum crystallite size.
Light rare earth-iron based magnetic alloy compositions and methods of processing them into permanent magnets are described in greater detail in U.S. Ser. Nos. 274,070; 414,936; 508,266; and 544,728 which are all to Croat, assigned to the assignee hereof and incorporated herein by reference. Neodymium and/or praseodymium-iron based magnetic alloys are particularly commercially significant because they exhibit magnetic energy products in the same class as samarium-cobalt permanent magnet alloys but at much lower cost.
In order to make bonded magnets from melt-spun alloy ribbon, it is necessary to break the friable ribbon into small pieces and then to compact the pieces under high pressure into desired magnet shapes.
U.S. Ser. No. 426,629 to Lee and Croat, which is assigned to the assignee hereof, relates to permanent magnets made from such alloy ribbon. A preferred method of making these magnets entails fracturing the friable alloy ribbons into particles small enough to fit in a compaction die, compacting the particles at a suitable pressure to achieve a magnetically isotropic, coherent compact with a density of at least about 75%, and then vacuum impregnating the voids of the compact with liquid epoxy. The epoxy is cured at an elevated temperature and any excess resin is machined away. While this "wet" process is suitable for laboratory use, it is not a preferred method for large scale production because it is not easy to handle catalyzed epoxy liquids and the impregnation process is relatively time consuming.
The concept of using organic and/or polymeric binders to make compacted particle magnets is not a new one. For example, it is a well known practice to mix a magnetizable alloy powder with a thermoplastic polymer that melts at low temperatures and then hot press or injection mold the mixture to make a magnet shape. Two injection mold the mixture to make a magnet shape. Two disadvantages of such processes are that the magnets produced are not suited for use at temperatures much above room temperature [i.e. at or above the glass transition temperature (T.sub.g) of the polymer] and that a substantial amount of nonmagnetic polymer (30 volume percent or more) dilutes the magnetic constituent. I have also experimentally determined that a polymeric bonding agent is much less effective as an oxidation barrier for a magnetic alloy at temperatures above its glass transition temperature. It is also known that the strength and shape retaining properties of a polymer are substantially reduced at temperatures above its T.sub.g.
Another known bonded magnet making practice entails dissolving a high melting polymeric constituent such as polycarbonate in a solvent; adding magnetic alloy powder to the solvent, and then adding a nonsolvent for the polymer to the mixture. The nonsolvent addition causes the alloy particles to precipitate out of solution, coated with the polymer. After the particles are dried, they can be hot pressed to coalesce the polymer coatings and form magnet shapes.
I believe that this method would be unsuited to working with rare earth-iron alloy powder because it would be very difficult to remove all the solvent from the precipitated polymer particles. Some solvent would be attracted to the alloy by ionic bonding, in a coprecipitation. Any solvent that remained would evaporate when the compact was finally heated thereby creating microscopic channels to the alloy surface. These channels would become vehicles for future oxidation of the rare earth-iron alloy and the accompanying degradation of its magnetic properties.
Attempts were made to precipitate thermosetting epoxy with a latent curing agent. This process resulted in a powder. When the dry-to-appearance precipitate powder mixed with alloy powder, compacted and then heated to cure the epoxy, the resin foamed in situ. The resultant product had poor strength and magnetic aging characteristics. The powder could not be dried at elevated temperature prior to compaction without prematurely activating the latent catalyst.
Because none of the conventional processes or chemical systems which were tried was found to be suitable for making polymer bonded rare earth-iron based magnets, a new approach was taken which resulted in the invention claimed in this patent.